One way in which power is supplied to a portable computer is by using a power adapter. The power adapter is coupled to an alternating current (AC) power source (such as an AC outlet) and converts an AC line voltage to a lower, direct current (DC) voltage suitable for use by the portable computer. The power adapter is typically a unit that is separate from the portable computer. A portable computer is designed to be moved. When a portable computer is moved to a new location, a power adapter is often brought to the new location along with the portable computer. As result, it is desirable to reduce the size and weight of the power adapter in order to facilitate the movement of the power adapter.
Typically, a portable-computer power adapter is designed to output a particular nominal output voltage for load currents up to a particular maximum current level. When the load current reaches or exceeds the maximum current level, the power adapter reduces the output voltage in order to attempt to prevent the load current from exceeding the maximum current level. Reducing the output voltage in this manner is referred to as “current limiting” the power adapter. Reducing the output voltage of the power adapter is typically undesirable (for example, such a reduction may result in the portable computer shutting down).
In one approach to avoiding having a power adapter enter such a current limiting state, a portable computer determines how much current the portable computer is drawing from the power adapter. The portable computer, in such an implementation, is designed and configured assuming that any power adapter used to power the portable computer will have a predetermined, fixed maximum current level (also referred to here as the “assumed maximum current level”). When the amount of current drawn by the portable computer exceeds the assumed maximum current level, the portable computer reduces the amount of power used by the portable computer. For example, in one implementation, the portable computer reduces the amount of power that is used for battery charging and/or reduces the clock frequency at which a central processing unit of the portable computer operates. This approach typically degrades the portable computer's performance (for example, by increasing the amount of time required to charge a battery and/or by reducing the speed at which a processor executes program instructions). However, if the portable computer is used with a power adapter that has a maximum current level that is greater than the assumed maximum current level used by the portable computer, the performance of the portable computer can be degraded unnecessarily (that is, in situations, where the current drawn by the portable computer is greater than the assumed maximum current level but less than the actual maximum current level of the power adapter).
Like reference numbers and designations in the various drawings indicate like elements.